1. Field of the Invention
The present invention relates generally to gas discharge lighting systems, and more specifically to electronic ballasts.
2. Description of the Prior Art
Half-bridge parallel-loaded series resonant converter (HB-PLSRC)circuit topologies are conventional for electronic ballasts. FIG. 1 illustrates a HB-PLSRC 10 where a direct current (DC) source 12 represents a DC bus. HB-PLSRC 10 includes a pair of switches 14 and 16, a resonant inductor 18, a resonant capacitor 20, a load 22 which represents a gas discharge lamp, and a pair of DC blocking capacitors 24 and 26. A DC current is thus prevented from flowing through the load 22. HB-PLSRC 10 is conventionally used in conjunction with an isolation transformer at the output, since there is no protection against a ground fault at the junction of load 22, resonant capacitor 20 and resonant inductor 18.
FIG. 2 illustrates a second HB-PLSRC 30 that includes active ground fault protection and a non-isolated output. A DC power source 32 is connected to the input. HB-PLSRC 30 includes a pair of switches 34 and 36, a resonant inductor 38, a resonant capacitor 40, a load 42 which represents a gas discharge lamp, and a pair of DC blocking capacitors 44 and 46. In HB-PLSRC 30, capacitor 44 limits any ground fault current at the junction of resonant inductor 38, resonant capacitor 40 and load 42.
Both HB-PLSRC 10 and HB-PLSRC 30 experience difficulties in electronic ballast applications where the circuit starts up without a load current. Gas discharge lamps, e.g., loads 22 and 42, do not immediately draw a current because the voltage applied across them must be raised high enough to start ignition. Until then, such lamps present a very high impedance. Thus, during start up or when the load is removed, much higher than normal currents can flow in the resonant components. Surge currents also flow through the DC blocking capacitors, since they are in series with the resonant inductor and capacitor tank circuit. Under typical operating conditions, the DC blocking capacitors are expected to carry currents that exceed those flowing through the load, because the resonating currents must also be supported. Surge currents will stress these components and high quality components with sufficient operating margins must be used to guarantee reliable, long-term operation. The DC blocking capacitors must also be sufficiently larger than their corresponding resonant capacitors so that their capacitive values do not substantially control the resonant frequency of the resonant inductor and resonant capacitor combination.